In known measuring devices for measuring the Sagnac-effect, light from a laser is coupled into a ring resonator in two opposite directions of propagation. When the measuring device is rotated about an axis perpendicular to the plane of the ring resonator the effective optical path lengths of the light waves propagating around are varied oppositely for one and the other opposite direction of propagation. This variation can be used for measuring the angular rate.
Such a measuring device having a passive ring resonator is described, for example, in U.S. Pat. No. 4,135,822.
With such measuring devices having a passive ring resonator the problem rises, that the light coupled out of the laser into the ring resonator in two opposite directions of propagation leaves the ring resonator through the respective opposite coupling-in paths and returns to the laser. Particularly when the emitted light frequency of the laser gets into resonance in the ring resonator, this proportion of the light coupled back can be considerable. This coupling-back of the light leads to the fact, that the laser responds to the resonance of the outer ring resonator. This is always unwelcome when the frequency of the laser is to be modulated independently, which is necessary in many measuring devices for measuring the Sagnac-effect. Therefore it is known to provide an "optical insulation" or some kind of "optical diode" between the laser and the ring resonator.
In the above mentioned U.S. Pat. No. 4,135,822, acousto-optical modulators (Bragg cells) are provided as such optical diodes.
It has been found that the desired optical diode effect can be achieved only incompletely. Particularly when semiconductor lasers are used, degrees of insulation smaller than 10.sup.-6 are required to let the laser be unaffected by its own light. Such a degree of insulation cannot be achieved with Bragg cells. Also by other methods, such a degree of insulation can only be achieved with considerable expenditure.